Add a new warning for the Python 3 porting checker, 'using-cmp-argument'.

This warning is emitted when the `cmp` argument for the `list.sort` or `sorted builtin`
is encountered, since it was removed in Python 3. This is part of the issue #376.

3 files changed, 57 insertions, 1 deletions

diff --git a/ChangeLog b/ChangeLog
index 9977d16..821e826 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,7 +1,7 @@
 ChangeLog for Pylint
 --------------------
 
-NOT RELEASED YET -- VERSION
+RELEASE DATE -- VERSION
     * Don't require a docstring for empty modules. Closes issue #261.
 
     * Fix a false positive with `too-few-format-args` string warning,
@@ -44,6 +44,10 @@ NOT RELEASED YET -- VERSION
     * Add support for combining the Python 3 checker mode with the --jobs
       flag (--py3k and --jobs). Closes issue #467.
 
+    * Add a new warning for the Python 3 porting checker, 'using-cmp-argument',
+      emitted when the `cmp` argument for the `list.sort` or `sorted builtin`
+      is encountered.
+
 
 2015-01-16 -- 1.4.1
 
diff --git a/pylint/checkers/python3.py b/pylint/checkers/python3.py
index 3ddf495..cf5e888 100644
--- a/pylint/checkers/python3.py
+++ b/pylint/checkers/python3.py
@@ -18,6 +18,7 @@ import re
 import tokenize
 
 import astroid
+from astroid import bases
 from pylint import checkers, interfaces
 from pylint.utils import WarningScope
 from pylint.checkers import utils
@@ -326,6 +327,13 @@ class Python3Checker(checkers.BaseChecker):
                   'Used when the filter built-in is referenced in a non-iterating '
                   'context (returns an iterator in Python 3)',
                   {'maxversion': (3, 0)}),
+        'W1640': ('Using the cmp argument for list.sort / sorted',
+                  'using-cmp-argument',
+                  'Using the cmp argument for list.sort or the sorted '
+                  'builtin should be avoided, since it was removed in '
+                  'Python 3. Using either `key` or `functools.cmp_to_key` '
+                  'should be preferred.',
+                  {'maxversion': (3, 0)}),
     }
 
     _bad_builtins = frozenset([
@@ -425,7 +433,38 @@ class Python3Checker(checkers.BaseChecker):
             else:
                 self.add_message('old-division', node=node)
 
+    def _check_cmp_argument(self, node):
+        # Check that the `cmp` argument is used
+        args = []
+        if (isinstance(node.func, astroid.Getattr)
+                and node.func.attrname == 'sort'):
+            inferred = utils.safe_infer(node.func.expr)
+            if not inferred:
+                return
+
+            builtins_list = "{}.list".format(bases.BUILTINS)
+            if (isinstance(inferred, astroid.List)
+                    or inferred.qname() == builtins_list):
+                args = node.args
+
+        elif (isinstance(node.func, astroid.Name)
+                and node.func.name == 'sorted'):
+            inferred = utils.safe_infer(node.func)
+            if not inferred:
+                return
+
+            builtins_sorted = "{}.sorted".format(bases.BUILTINS)
+            if inferred.qname() == builtins_sorted:
+                args = node.args
+
+        for arg in args:
+            if isinstance(arg, astroid.Keyword) and arg.arg == 'cmp':
+                self.add_message('using-cmp-argument', node=node)
+                return
+
     def visit_callfunc(self, node):
+        self._check_cmp_argument(node)
+
         if isinstance(node.func, astroid.Getattr):
             if any([node.args, node.starargs, node.kwargs]):
                 return
diff --git a/pylint/test/unittest_checker_python3.py b/pylint/test/unittest_checker_python3.py
index 825e827..bac3f4a 100644
--- a/pylint/test/unittest_checker_python3.py
+++ b/pylint/test/unittest_checker_python3.py
@@ -384,6 +384,19 @@ class Python3CheckerTest(testutils.CheckerTestCase):
         with self.assertNoMessages():
             self.walk(node)
 
+    def test_using_cmp_argument(self):
+        nodes = test_utils.extract_node("""
+        [].sort(cmp=lambda x: x) #@
+        a = list(range(x))
+        a.sort(cmp=lambda x: x) #@
+
+        sorted([], cmp=lambda x: x) #@        
+        """)
+        for node in nodes:
+            message = testutils.Message('using-cmp-argument', node=node)
+            with self.assertAddsMessages(message):
+                self.checker.visit_callfunc(node)
+
 
 @python2_only
 class Python3TokenCheckerTest(testutils.CheckerTestCase):